Flow meters, which may be used to measure the flow of a fluid medium, are known from the related art. For example, these are equipped to detect velocities of flow, mass flows or volume flows, or combinations of the aforementioned measured variables of the fluid medium. For example, the flow meters may be used in a flow pipe.
A known measurement principle is based on the use of ultrasonic signals, which are emitted into the flowing fluid medium. One, two or more ultrasonic transducers may be used, emitting ultrasonic signals into the fluid medium and/or receiving them from the fluid medium, namely at an angle such that the ultrasonic signals have at least one propagation component in or against a main direction of flow of the flowing medium. For example, the flow of the fluid medium may then be determined from transit time differences for the propagation into and against the direction of flow. Other methods are based, for example, on reflection of ultrasonic signals and/or pulse technologies. Flow meters in general are known from the related art, e.g., from German Patent No. DE 10 2004 061404 or European Patent No. EP 0605944. For example, it is also possible to go back to the measurement principles disclosed in these documents within the framework of the present invention to infer the flow of the fluid medium from ultrasonic signals.
However, known ultrasonic transducers usually have a limited spectrum of application. For example, many ultrasonic transducers known from the related art have a sensitive response to hot and/or aggressive media, for example, corrosive media, acids, oils or the like. For example, at the present time there are no known measurement methods which do justice to all the requirements of this application for measuring exhaust gas mass flows in internal combustion engines, in particular in the automotive field. Various measurement principles have been tested.
However, known methods are not usually suitable for the conditions occurring in exhaust gas mass flows. For example, high temperatures, usually temperatures significantly above 100° C., occur in exhaust gas mass flows. In addition, contaminants also occur there, for example, in the form of soot, acids, water, oil or other contaminants. Another unfavorable ambient condition is that a high degree of heterogeneity prevails in exhaust gas mass flows. In particular, there may be oscillations of the gas column having frequencies in the range between 10 and 1000 Hz. Traditional flow meters are not usually suitable for such boundary conditions.